1. Field of the Invention
The present invention relates generally to non-destructive examination of heat exchanger tubing and more particularly to an eddy current probe that exhibits reduced friction and improved centering as it travels within the heat exchanger tubing.
2. Related Art
Steam generators used in nuclear reactor power plants are very large heat exchangers where heat from a primary fluid heated by a nuclear reactor is transferred to a secondary fluid which is converted into steam and used to drive a turbine generator. Steam generators are housed inside a tall, generally cylindrical steel shell. A large number of U-shaped heat exchanger tubes are enclosed in the shell and have their ends inserted in holes formed in a horizontal tube sheet or plate near the bottom of the steel shell. The tubes are used to convey the primary fluid which has been heated in the nuclear reactor. The secondary fluid or feedwater used to generate the steam is introduced into the steam generator in such a manner that the secondary fluid flows around the outside of the heated tubes thereby converting much of the secondary fluid into steam which is allowed to exit the steam generator through an outlet nozzle at the top of the steel shell.
In the past, steam generator tubing in nuclear plants have been exposed to extreme operating conditions and were susceptible to stress corrosion cracking, mechanical wear, wall thinning and pitting. To address this susceptibility, a number of techniques have been developed to inspect steam generator tubing for degradation prior to tubing failure in order to prevent leakage of the primary radioactive coolant into the secondary side which would result in forced outages. Steam generator tubing has been most commonly inspected using a variety of eddy current methods, most involving probes which were inserted into the tubes from the underside of the tube sheet on the primary side of the steam generator. The probes are inserted through a steam generator manway in the lower hemispherical inlet and outlet side of the steam generator below the tube sheet and into the tube sheet whereby the corresponding tubes are mapped by inserting the probes up through the tubes.
One type of eddy current probe that is used for this purpose is the “Bobbin”-type, wherein two coils of copper wire are wound circumferentially around a relatively rigid core to create the test coil. Although widely and relatively successfully used, the relatively rigid Bobbin-type probes are difficult to push through tubes that have bends.
Another type of eddy current probe is a rotating pancake coil probe. The rotating pancake coil probe is ordinarily mounted on a motorized sheath which allows the coil to be simultaneously rotated and translated through the tube thereby developing a helical scan of the tube surface. The pancake eddy current coil axis is normal to the tube inside diameter surface and generally the coil is mounted in an articulating mechanism that allows the coil to follow the inside diameter surface contour, and to maintain a relatively constant coil liftoff.
A third type of eddy current probe employs as many as 40 individual coils that are arranged circumferentially around the probe. Each of the coils provides its own individual output of the opposing inside surface of the heat exchanger tube as the probe is translated axially through the tube.
Generally, each of the foregoing probes needs to be centered as they move through the interior of the heat exchanger tube. Typically, the probes are centered employing compliant pads that extend out radially at spaced circumferential locations around the probe at generally two spaced axial positions. Though highly accurate, the eddy current method of inspecting steam generator tubing is relatively slow and preferably the probes, which are typically pushed along the length of the tubing by a flexible shaft, should be inserted at a constant rate. Side loads developed by the pushing shaft as the probe traverses bends in the tubing and to a lesser extent side loads due to off-center shaft positioning in the straight lengths can adversely impact the centering of the probe or its forward progress.
Accordingly, an improved method of centering the probe is desired that can better navigate bends and resist the effects of side loads from the shaft.
Additionally, an improved method of centering the probe is desired that can reduce resistance to the probe's forward progress as it is pushed through the steam generator tubing.